basic logic gates

3/28/2015 Basic Logic Gates

http://www.ee.surrey.ac.uk/Projects/CAL/digital-logic/gatesfunc/index.html 1/7

Basic Gates and Functions

Introduction

Truth Tables

Logic Gates

AND gate

OR gate

NOT gate

NAND gate

NOR gate

EOR gate

ENOR gate

Example

Problem

Multiple Input Gates

Tutorials with LabVIEW Simulations

Gates and Functions Quiz

Introduction

Boolean functions may be practically implemented by using electronic gates. The

following points are important to understand.

Electronic gates require a power supply.

Gate INPUTS are driven by voltages having two nominal values, e.g. 0V and

5V representing logic 0 and logic 1 respectively.



The OUTPUT of a gate provides two nominal values of voltage only, e.g. 0V

and 5V representing logic 0 and logic 1 respectively. In general, there is only

one output to a logic gate except in some special cases.

There is always a time delay between an input being applied and the output

responding.

Truth Tables

Truth tables are used to help show the function of a logic gate. If you are unsure about truth tables

and need guidence on how go about drawning them for individual gates or logic circuits then use the

truth table section link.

Truth Tables

Logic gates

Digital systems are said to be constructed by using logic gates. These gates are the AND, OR,

NOT, NAND, NOR, EXOR and EXNOR gates. The basic operations are described below with

the aid of truth tables.

AND gate

The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are

high. A dot (.) is used to show the AND operation i.e. A.B. Bear in mind that this dot is

sometimes omitted i.e. AB



OR gate

The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs

are high. A plus (+) is used to show the OR operation.

NOT gate

The NOT gate is an electronic circuit that produces an inverted version of the input at its output. It is also

known as an inverter. If the input variable is A, the inverted output is known as NOT A. This is also

shown as A', or A with a bar over the top, as shown at the outputs. The diagrams below show two ways

that the NAND logic gate can be configured to produce a NOT gate. It can also be done using NOR

logic gates in the same way.

NAND gate

This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate. The

outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate



with a small circle on the output. The small circle represents inversion.

NOR gate

This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate. The outputs

of all NOR gates are low if any of the inputs are high.

The symbol is an OR gate with a small circle on the output. The small circle represents

inversion.

EXOR gate

The 'Exclusive-OR' gate is a circuit which will give a high output if either, but not both, of

its two inputs are high. An encircled plus sign ( ) is used to show the EOR operation.

EXNOR gate



The 'Exclusive-NOR' gate circuit does the opposite to the EOR gate.

It will give a low output if either, but not both, of its two inputs are

high. The symbol is an EXOR gate with a small circle on the output. The

small circle represents inversion.

The NAND and NOR gates are called universal functions since with either one the

AND and OR functions and NOT can be generated.

Note:

A function in sum of products form can be implemented using NAND gates by

replacing all AND and OR gates by NAND gates.

A function in product of sums form can be implemented using NOR gates by

replacing all AND and OR gates by NOR gates.

Table 1: Logic gate symbols

Table 2 is a summary truth table of the input/output combinations for the NOT gate

together with all possible input/output combinations for the other gate functions. Also

note that a truth table with 'n' inputs has 2n rows. You can compare the outputs of



different gates.

Table 2: Logic gates representation using the Truth table

Example

A NAND gate can be used as a NOT gate using either of the following wiring

configurations.

(You can check this out using a truth table.)

Problem

Draw the circuit diagrams like the ones in the example above to show how a NOR

gate can be made into a NOT gate.

Click here for answers.




There are also multiple input gates if you want to know more about them then click on

the link below.


Tutorials with LabVIEW simulations

Here are some tutorials using LabVIEW simulations to show the gate functions and

some of the different ways that gates can be configured.

Tutorials and Simulations

Gates and Functions Quiz

There is a quiz available to test what you have learned so far. quiz

To submit your questions and queries please click here:

Composed by Wale Sangosanya 1997

Updated by David Belton - April 98

Updated by Richard Bigwood 2005

basic logic gates

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